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The correct phrase is "a once-in-a-lifetime experience." The article "a" is needed before "once-in-a-lifetime" to make it grammatically correct.
This job is a once in a lifetime opportunity.
A once in a lifetime trip can be expensive.
That is the correct spelling of "once in a lifetime."
a once in a lifetime opportunity
Winning the jackpot in the lottery is a once in a lifetime event.